1. Field of the Invention
The present invention relates to a technique for calculating the depth of an object from a plurality of images captured by changing image capturing conditions.
2. Description of the Related Art
A depth from defocus (DFD) method such as described in Patent Literature 1 has been suggested as a method for acquiring a distance in the depth direction of an object in an image from a two-dimensional image acquired with an imaging apparatus. In the DFD method, a plurality of images with different blur are acquired by controlling the image capturing conditions (can be also referred to as image capturing parameters) of an image capturing optical system, and depth information on the object is acquired by calculating the quantity of blur or correlation value of the blur between the plurality of images by using a depth measurement object pixel and surrounding pixels thereof.
Since the quantity of blur or correlation value of the blur varies with the depth from the focal position of the image capturing optical system to the object, the depth of the object can be calculated using this relationship. In the depth measurements performed by the DFD method, the depth can be calculated with a single image capturing optical system. The resultant advantage is that the system can be easily incorporated into a commercial imaging apparatus such as a digital camera.
Patent Literature 2 describes a depth measurement device that calculates the object depth from the correlation value of the blur computed for each processing object region by using the DFD method. In this device, a weighting coefficient is set for the correlation value of the blur in pixel units for each pixel in the processing object region, and finding the weighted average of the correlation value of the blur on the basis of the weighting coefficients. As a result, highly accurate depth measurements are performed even when the object depth is spatially varied.    Patent Literature 1: Japanese Patent Application Publication No. H1-167610    Patent Literature 2: Japanese Patent Application Publication No. 2010-016743